Cross‐Over Infrared Spectroscopy: A New Tool for the Remote Determination of Olivine Composition

Abstract

Olivine ([Mg,Fe] 2 SiO 4) has been remotely detected on numerous solar system bodies, but its composition has been only partially constrained using visible-near infrared (VNIR, 0.5-3 μm) and midinfrared (MIR, 8-15 μm) spectra. Meanwhile, the spectral character of olivine in the "cross-over" region of the infrared (4-8 μm) remains generally unexplored. We examine the relationship between olivine Mg# (molar Mg/[Mg + Fe] × 100) and the wavelength positions of two strong, discrete overtonecombination bands at~5.6 and~6.0 μm. In reflectance and thermal emission spectra measured for 47 synthetic and natural olivine samples that span the Mg-Fe solid solution, we find that the 5.6 and 6.0 μm bands shift systematically to longer wavelengths by 0.11 and 0.10 μm, respectively, with increasing Fe content. Our results indicate that olivine Mg# may be determined within ±10 mol% forsterite using the positions of these two bands, making "cross-over" region spectroscopy a potentially powerful remote sensing tool.

A Micro Mid‐Infrared Spectroscopic Study of Chang’e‐5 Sample

“…The prominent RB peaks and strong overtone and combination bands in olivine spectra are often used to quantify its chemical composition. The positions of the 5.6 and 6.0 μm absorption features in the cross‐over region and the five RB peaks numbered as 1–5 in Figure 3a can be used to derive the Fo (molar Mg/[Mg + Fe] × 100) of olivine (Hamilton, 2010; Kremer et al., 2020). The corresponding band positions of those olivine samples mentioned above are listed in Table 1.…”

“…These band positions were determined by directly identifying the corresponding wavenumber of the local reflectance minimum (for the CF, 5.6, and 6.0 μm bands) or maximum (for the RB). Following the empirical functions (Equations 1–2 in Kremer et al., 2020 and Table 4 in Hamilton, 2010), we obtained the predicted Fo of those samples (Table 2). Hereafter, we will refer the former as the Kremer’s method and the latter as the Hamilton’s method.…”

“…The whole set of Y0-Y7 spectra are shown in Figure S3 in Supporting Information S1. (c) and (d) Show the Fo derived using the empirical equations proposed by Kremer et al (2020) and Hamilton (2010), respectively. Note that points with unreasonable derived Fo values, that is, beyond the range [0, 100], are not shown.…”

“…Furthermore, the extraterrestrial materials may not have the same set of compositions compared to their terrestrial counterparts. For example, natural terrestrial olivines are usually more Mg‐rich than lunar olivine (Kremer et al., 2020). In addition, structural differences caused by space weathering may also affect the spectral features of regolith grains on airless bodies (e.g., Yang et al., 2017).…”

“…In addition, the “cross‐over” region of the infrared (4–8 μm) contains overtones and combinational bands of the fundamental lattice vibrations and may also be used to characterize mineral compositions (Salisbury et al., 1991). For instance, the 5.6 and 6.0 μm bands of olivine systematically shift to shorter wavelengths with increasing Mg content (Kremer et al., 2020).…”